The present invention relates generally to digital receivers, and, more particularly, to a system, and associated method therefor, for weighting a signal received by the receiver to associate thereby a confidence level with the received signal.
A communication system which transmits information between two locations includes, at a minimum, a transmitter and a receiver wherein the transmitter and the receiver are interconnected by a transmission channel upon which an information signal (which contains information) may be transmitted.
In one type of communication system, a radio communication system, the transmission channel is comprised of a radio-frequency channel which interconnects the transmitter and the receiver. To transmit an information signal (referred to as a baseband signal) upon the radio-frequency channel, the information signal must be converted into a form suitable for transmission thereof upon the radio-frequency channel.
Such a conversion of the information signal into a signal suitable for transmission upon the radio-frequency channel is accomplished by a process referred to as modulation wherein the information signal is impressed upon a radio-frequency electromagnetic wave. The radio-frequency electromagnetic wave is a sinusoidal wave of a frequency of a value within a range of values of frequencies defining the radio-frequency channel. The radio-frequency electromagnetic wave is commonly referred to as a carrier signal, and the radio-frequency electromagnetic wave, once modulated by the information signal, is referred to as a modulated, information signal. The modulated, information signal comprises a communication signal which may be transmitted through free space.
The information content of the resultant, modulated, information signal occupies a range of frequencies, centered at, or close to, the frequency of the carrier signal. Because the modulated, information signal may be transmitted through free space upon the radio-frequency channel to transmit thereby the information content of the information signal between the transmitter and the receiver of the communication system, the transmitter and receiver need not be positioned in close proximity with one another.
Various modulation techniques have been developed to modulate the information signal upon the carrier signal to permit such transmission upon the radio-frequency transmission channel. Such modulation techniques include: amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), frequency-shift keying modulation (FSK), phase-shift keying modulation (PSK), and continuous phase modulation (CPM). One type of continuous phase modulation is Gaussian minimum shift keying modulation (GMSK).
The receiver which receives the modulated, information signal contains circuitry to detect, or to recreate otherwise, the information signal modulated upon the carrier signal. Typically, the circuitry of the receiver includes circuitry (sometimes consisting of several stages) to convert downward in frequency the modulated, information signal received by the receiver in addition to the circuitry required to detect, or to recreate otherwise, the modulated, information signal. The process of detecting or recreating the information content of the modulated, information signal is referred to as demodulation and such circuitry for performing demodulation is referred to as demodulation circuitry or a demodulator. Sometimes, both the down-conversion circuitry and the demodulator are together referred to as demodulation circuitry.
A plurality of modulated, information signals may be simultaneously transmitted as long as the simultaneously-transmitted, modulated, information signals are comprised of carrier signals of dissimilar frequencies, and the resultant, modulated, information signals do not overlap in frequency. More particularly, the frequencies of the carrier signals of the simultaneously-transmitted, modulated, information signals must be separated in frequency to prevent the information content of the modulated, information signal (i.e., the modulation spectrum) from overlapping with corresponding signals modulated upon carrier signals of frequencies of values proximate thereto.
A receiver includes tuning and other filter circuitry to pass received signals of only certain frequencies to down-convert in frequency and to demodulate only signals within certain bandwidths. Such tuning and other filter circuitry form frequency passbands for passing only those signals having frequency components within the frequencies defined by the passbands of the tuning and other filter circuitry.
The broad range of frequencies of which the carrier signal may be comprised and upon which the information signal may be modulated is referred to as the electromagnetic frequency spectrum. Regulatory bodies have divided the electromagnetic frequency spectrum into frequency bands, each of which defines a range of frequencies of the electromagnetic frequency spectrum. The frequency bands have been further divided into channels, such channels form the transmission channels of a communication system. The regulatory bodies regulate transmission of radio-frequency signals in certain ones of the frequency bands of the electromagnetic frequency spectrum to minimize interference between simultaneously-transmitted, modulated, information signals.
For instance, portions of a 100 MHz frequency band extending between 800 MHz and 900 MHz are allocated in the United States for radiotelephone communication. Radiotelephone communication, may, for example, be effectuated by a radiotelephone utilized in a cellular, communication system. Such a radiotelephone contains circuitry to permit simultaneous generation and reception of modulated, information signals, thereby permitting two-way communication between the radiotelephone and a remotely-located receiver.
In general, a cellular, communication system is created by positioning numerous base stations at spaced-apart locations throughout a geographical area. Each base station contains circuitry to receive modulated, information signals transmitted by one, or many, radiotelephones, and to transmit modulated, information signals to the one, or many, radiotelephones. Because both a base station and a radiotelephone permits both transmission and reception of modulated, information signals, two-way communication between a radiotelephone and a base station is permitted.
The position at which each of the base stations of the cellular, communication system is located is carefully selected so that at least one base station is within the transmission range of a radiotelephone positioned at any location throughout the geographical area. Because of the spaced-apart nature of the positioning of the base stations, portions of the geographical area throughout which the base stations are located are associated with individual ones of the base stations. Portions of the geographical area positioned proximate to each of the spaced-apart base stations define "cells" wherein the plurality of cells, each associated with a base station, together form the geographical area encompassed by the cellular, communication system. A radiotelephone positioned within the boundaries of any of the cells of the cellular, communication system may transmit, and receive, modulated, information signals to, and from, at least one base station.
Typically, communication between a radiotelephone and a base station includes both data signals and voice signals which are alternately, and also simultaneously, transmitted upon one or more transmission channels. Data transmitted between the base station and the radiotelephone includes instructions to cause the radiotelephone to receive and to transmit signals upon particular radio-frequency channels. Signals are also transmitted between the base station and the radiotelephone for purposes of synchronization to ensure that transmission of the modulated, information signal from a particular base station is received by a particular radiotelephone.
Increased usage of cellular, communication systems has resulted, in many instances, in the full utilization of every available transmission channel of the frequency band allocated for cellular, radiotelephone communication. As a result, various ideas have been proposed to utilize more efficiently the frequency band allocated for radiotelephone communications. More efficient utilization of the frequency band allocated for radiotelephone communication increases the transmission capacity of a cellular, communication system.
One such means by which the transmission capacity of the cellular, communication system may be increased is to utilize digital modulation techniques. When an information signal is converted into digital form, a single transmission channel may be utilized to transmit, sequentially, more than one information signal. Because more than one information signal may be transmitted upon a single transmission channel, the transmission capacity of an existing frequency band may be increased by a multiple of two or more.
Typically, an analog information signal is first converted into digital form (such as, for example) by an analog-to-digital converter), and then encoded by some coding technique. Then, the encoded signal is modulated to transmit thereby the information signal of the radio-frequency channel. A modulation technique which may be advantageously utilized to transmit such a digital signal is the aforementioned Gaussian minimum shift keying (GMSK) modulation. Such a modulation technique is discussed more fully in a text entitled "Digital Phase Modulation" J. B. Anderson, T. Aulin, and C. E. Sundberg, Published by Plenum Press, Copyright 1986.
Transmission of any signal upon a radio-frequency transmission channel is susceptible to error as a result of noise and other interference occurring as a result of transmission of a signal upon the transmission channel. Noise is caused, for example, by the presence of spurious and other transient signals. Other interference occurs, for example, as a result of reflection of transmitted signals off of both man-made and natural objects. Such reflection of transmitted signals results in the same signal being received by a receiver at differing times (referred to as signal delay) corresponding to the path by which the signal is transmitted to the receiver. For instance, a transmitted signal which is reflected off of an object resulting in an increase in path length between the transmitter and receiver of four fifths of a mile results in a four microsecond delay as a result of the increased path length. Path lengths of increase distances result, accordingly, in delays of increased time periods. Because of such signal delay, the signal received by the receiver is actually the summation of a single transmitted signal which is transmitted to the receiver over a multiplicity of paths. The transmission channel is, therefore, oftentimes referred to as a "multipath" channel. Such signal delay results in signal interference.
When the transmitted signal is a digitally-encoded signal, such interference caused as a result of transmission of the signal upon a multipath channel results in a type of interference referred to as intersymbol interference. As the digitally-encoded signals to be utilized in a cellular, communication system are transmitted at a bit rate of in excess of 270 kilobits per second, even a delay as small as the four microsecond delay, mentioned hereinabove, can result in significant amounts of intersymbol interference.
Because the transmitted, digitally-encoded signal is encoded to increase the redundancy of the transmitted signal, some of the errors occurring as a result of such intersymbol interference (as well as errors occurring as a result of other noise) are removed during a receiver decoding process of the signal received by the receiver. However, because each error which occurs as a result of intersymbol interference which results in an incorrectly-decoded signal reduces the quality of the communication between the transmitter and the receiver, it is highly desirable to detect the existence of such errors, or to provide an indication of the likelihood of such error.
Equalizer circuitry, both of software and hardware implementations, are known, and are utilized to correct for the effects of transmission of a signal upon a multipath channel. For instance, U.S. patent application Ser. No. 422,177, filed Oct. 13, 1989, by David E. Borth, Phillip D. Rasky, and Gerrald P. Labedz, entitled "Soft Decision Decoding With Channel Equalization", and U.S. patent application Ser. No. 442,971, filed Nov. 29, 1989, by David E. Borth, entitled "Soft Trellis Decoding", both disclose systems utilizing equalizer circuitry to correct for intersymbol interference caused by transmission of a signal upon a multipath channel. The channel equalizer utilized in each of the just-mentioned disclosures is formed of a maximum likelihood sequence estimator (MLSE). An MLSE is operative to estimate sequences of a transmitted signal responsive to the signal actually received by the receiver. In general, the MLSE (as well as an equalizer of other design) is operative to remove the intersymbol interference generated as a result of transmission of a signal upon a multipath channel. The signal generated by the MLSE is then applied to the receiver decoder circuitry. The decoder circuitry decodes the equalized signal (and in the instance of an MLSE, an estimated signal) to remove redundancies in the signal intentionally created by a transmitter encoding circuitry.
Operation of an MLSE is described more fully in a paper entitled, "Adaptive Maximum-Likelihood Receiver for Carrier-Modulated Data-Transmission Systems", by G. Ungerboeck, in the IEEE Transactions on Communications, volume COM-22, pages 624-635, May. 1974. The MLSE disclosed therein is comprised of a matched filter which is supplied with the signal received by the receiver (once down-converted and demodulated by down conversion and demodulation circuitry), and a Viterbi algorithm which is supplied with the signal passed by the matched filter.
The Viterbi algorithm forms a trellis of possible paths, a particular matrix of which is utilized to form a sequence (or stream) of data. Because the signal at the output of the Viterbi algorithm is a sequence of data bits, the output of the MLSE is a hard decision signal (i.e., comprised of a sequence of bits of digital values).
Although the transmitted signal is a digitally-encoded signal, the information signal, once modulated upon the sinusoidal, carrier signal, is an analog signal. As the possible values of which the digitally-encoded signal actually transmitted may be comprised are finite (for instance, when the digitally-encoded signal is a binary signal, the digitally-encoded signal may be of only two possible values) the output of the MLSE is of a corresponding number of possible values. The Viterbi algorithm of the MLSE converts the digitally-encoded signal applied thereto in analog form into the sequence of data. Such conversion, and resultant use of only the resultant, data sequence generated by the Viterbi algorithm does not fully utilize the entire information of the signal supplied to the MLSE.
In particular, the signal actually received by the MLSE, and passed by the matched filter forming a portion thereof, may be compared with the data sequence generated by the Viterbi algorithm of the MLSE. Such a comparison may be utilized to provide an indication of the level of confidence to be associated with the signal supplied to the receiver decoder circuitry to indicate thereby the level of confidence with which the received signal is believed to be accurate. Such an indication could be utilized to minimize further the errors resulting from intersymbol interference caused by transmission of a signal upon a multipath channel, and the resultant degradation in communication quality as a result thereof.
What is needed, therefore, is a system to utilize best the signal received by a digital receiver to minimize best the errors caused by noise and/or intersymbol interference of a signal transmitted upon a multipath channel.